Slowdown of the rise in the optimum temperature of photosynthetic productivity under future global warming

The optimum air temperature for photosynthetic ecosystem productivity (Topt) determines the annual maximum of terrestrial carbon uptake. Previous studies have quantified Topt and explored its acclimation in response to climatic warming. However, there remains uncertainty regarding the extent to which Topt has changed globally in recent decades and how it might evolve in the future. Our analysis, using both satellite- and ground-based datasets, reveals a slight increase in Topt over the past two decades (+0.14°C, 2000-2019), contrasting with a significant 0.46°C rise in maximum global growing-season temperature (Tmax). The lack of change in Topt was attributed to an insignificant trend of Tmax in cold areas and to drought inhibiting thermal acclimation of photosynthesis in temperate and arid regions. If global surface temperatures exceeded pre-industrial levels by 2°C, the mean Tmax over the globe was predicted to increase slightly more than Topt (1.5 vs. 1.2°C), with the gap widening under a 4°C temperature increase (4.1 vs. 3.4°C). In the +4°C scenario, soil drought dominated the widespread decline in photosynthetic acclimation across tropical and temperate vegetation, slowing the rate of Topt increase at the end of the century (+0.04°C, 2080-2099). Conversely, increasing atmospheric CO2 concentrations failed to have significant effects on Topt and its acclimation. These findings imply that the absorption of atmospheric CO2 by terrestrial vegetation, and subsequent carbon sequestration, may be further hampered by the limited acclimation capacity of Topt to rising global temperatures.